The present invention relates to a method of forming a multi-piled bump on an electrode of a semiconductor chip, etc.
A semiconductor chip, which will be flip chip-connected, has stud bumps formed on aluminum pads, and the semiconductor chip is mounted, for example, on a substrate of a printed circuit board by electrically connecting the stud bumps to electrodes of the substrate. Generally, the bump is formed on the pads of the semiconductor chip by the steps of: sparking and melting a front end of a metal wire so as to form a metal ball; and pressing the metal ball onto the pad, by a capillary, with applying ultrasonic vibrations so as to connect the metal ball with the pad.
Each of the bumps is formed by one metal ball, and its height is determined on the basis of a size of the metal ball formed at the front end of the metal wire, a pressing force applied to the metal ball to connect with the pad and a power of the ultrasonic vibrations applied to the metal ball.
In the case that high bumps are required due to parts arrangement on the substrate, limitation of mounting the semiconductor chip, etc., diameters of the ball metals are made greater so as to make the bumps higher. However, width of the bumps are made greater in proportion to the diameters thereof, so separations between the bumps cannot be made narrower. In another method, the pressing force applied to each metal ball is made small so as not to make the metal ball thicker. In this method, however, connecting strength between the bumps and the pads are lowered, so that the bumps cannot be formed stably. Further, if the power of the ultrasonic vibrations is lowered so as to make the bumps higher, the same problem occurs.
To solve the problems, a method of forming multi-piled stud bumps on pads of a semiconductor chip is disclosed in Japanese Patent Gazette No. 2002-43357. In the method, 3–6 stud bumps (metal balls) are piled by a capillary, which is used for wire bonding, so as to form each multi-piled bump.
By employing the multi-piled bumps, the bumps can be made higher without making their diameters greater. Therefore, separations between the multi-piled bumps can be narrower. Further, the height of the multi-piled bump can be easily adjusted by selecting number of piling the metal balls. However, the metal balls having small diameters cannot be stably and securely piled by merely adjusting boding conditions.